1. Field of the Invention
The invention concerns an magnetic resonance apparatus of the type having a cylindrical gradient coil with x- and y-coils directed asymmetrically relative to a z-direction (running along a patient receptacle) of a shim coils for generation of magnetic fields serving for the homogenization coordinate system, as well as a number of a basic magnetic field generated by means of a cylindrical basic field magnet.
2. Description of the Prior Art
A magnetic resonance apparatus serves for the high-resolution acquisition of images of an examination subject using magnetic fields. Various magnetic fields are generated for this purpose. A static basic magnetic field is generated using a (normally cylindrical) basic field magnet, whereby the field strength normally being between 0.2 and 10 T. Furthermore, a radio-frequency excitation field (B1 field) for excitation of the nuclear spins is generated using a radio-frequency coil. For spatial resolution a gradient field with three separate field components in the three spatial axes is also generated using a gradient coil. Because the basic magnetic field is not entirely homogeneous, but rather exhibits or is subject to disruptions, shim coils are also used that respectively generate correction magnetic fields with which the homogeneity of the basic magnetic field is adjusted. Typically five shim coils are used that respectively generate a magnetic field that corresponds to a specific term of a spherical function expansion of the second order of the basic magnetic field. These five terms or field terms are typically explicitly determined by a magnetic field measurement so that each shim coil can be correspondingly adjusted so the shim magnetic field generated by each shim coil corrects the respective term-related inhomogeneity.
The gradient coils (thus the x-coil, y-coil and z-coil) are typically arranged or wound or directed symmetrically around the periphery and symmetrically in the z-direction. To avoid couplings between the shim coils and the gradient coils due to the respectively generated fields, the respective shim coils are also formed symmetrically. Given special gradient coils (for example for head imaging), the symmetry of the x- and y-gradient coils relative to the z-direction is destroyed due to the specific shaping of such gradient coils that, with regard to the patient body to be acquired, exhibit frontal broadenings or recesses. This means that the x- and y-gradient coils are directed asymmetrically relative to the z-direction. However, as before the associated shim coils are executed symmetrically, which (as it now turns out) leads to considerable coupling inductances of up to approximately 40 μH. Given a slew rate of, for example, 4 MA/s, this coupling inductance leads to an induced voltage of approximately 160 V in the coupled shim coil, meaning that a considerable alternating voltage is injected (induced) into the shim coil. This represents a significant load for the shim amplifier, which must still deliver the constant shim current in a stable manner for the respective coil while this injected alternating voltage is present.